Aluminide coatings are diffused coatings widely used to protect metallic substrate surfaces, such as nickel, cobalt, iron and copper alloys. Aluminide coatings are based on intermetallic compounds formed when nickel and cobalt react with aluminum at the substrate's surface. An intermetallic compound is an intermediate phase in a binary metallic system, having a characteristic crystal structure enabled by a specific elemental (atomic) ratio between the binary constituents.
Aluminum-based intermetallic compounds (i.e., aluminides) are resistant to high temperature degradation. As a result, they have emerged as preferred protective coatings. The protective aluminide coatings provide high temperature oxidation and corrosion protection for various end-use applications. These coatings are particularly effective for protection of aerospace components, such as gas turbines engines.
Gas turbine engines include various components such as blades, vanes and combustor cases. The components are usually made from nickel and cobalt alloys. During operation, these components are typically located in the hot section of the turbine and exposed to the hot gases from the turbine combustion process where oxidation and corrosion can occur. In particular, oxidation and corrosion reactions at the surface of the component parts can cause metal wastage and loss of wall thickness. The loss of metal rapidly increases the stresses on the respective component part and can result in part failure. Aluminide coatings are thus applied to these component parts to protect the structural integrity of the part by providing resistance against oxidation and corrosion.
Careful dimensional tolerances imposed on parts during manufacture must also be maintained during the aluminide coating process. The aluminide coating process involves heating a metallic substrate surface in the presence of an aluminum containing source material. The aluminum-containing source material includes a halide activator and an aluminum “donor” or source alloy. As used herein and throughout the specification, it should be understood that the term “donor” and “source” are used interchangeably. When the material is heated, the donor alloy and activator react to generate an aluminum vapor. The vaporized aluminum transfers to the metallic substrate surface and diffuses into the metal surface creating a protective outer layer of metal aluminum alloy. The aluminum reacts with the substrate to form intermetallic compounds. An additive layer containing the aluminum is also formed.
The aluminide coating process generally involves coating the external and internal sections of a component. One type of aluminide coating is typically used to coat the external surfaces and a second type of aluminide is used to coat the internal section. Uneven or excessively thick diffusion coating layers to the parts can effectively act to reduce wall thickness and hence the part's strength. Furthermore, excessively thick aluminide coatings, especially at leading and trailing edges of turbine blades where high stresses mostly occur, can result in fatigue cracking.
Moreover, the components are typically constructed with hollow core passages for transporting internal cooling air. As a result, the internal surfaces of the hollow components must be coated in a way that not only produces uniform thickness coatings, but leaves unobstructed the cooling air passages along the internal surfaces. Advancements in the aerospace industry have led to gas turbine components which are designed with increasingly complicated geometries along surfaces of the internal cavities, thereby making the ability to uniformly coat such surfaces more challenging than previously encountered.
One technique for the application of aluminide coating onto internal surfaces of the hollow components relies on the direct application of donor and activator to the internal surfaces utilizing a pack of aluminizing powder. The pack technique involves utilizing aluminum powder, which is mixed with an activator such as aluminum fluoride or ammonium fluoride. The part to be coated is immersed into this powder with the activator in a manner to ensure the part is completely surrounded by the aluminum-based powder. The aluminum-based powder is also forced into the internal sections of the part, and thereafter heated to melt and diffuse the powder into the surface. However, an undesirable residual coating, some of which may be referred to as “bisque” in the industry, can be difficult to remove from the cooling air holes and internal passages. “Bisque” as used herein and throughout the specification is intended to include oxidized material including scale (e.g., AlxOy); donor material constituents (e.g., halide activators and donor source materials); and by-products resulting from secondary reactions of the donor material constituents, including that of the halide-containing activator with atmospheric gases (e.g., AlxNy)—all of which are formed during the coating process and which become undesirably incorporated into the resultant aluminide coating. Bisque can cause restriction of air flow. As a result, the part must be scrapped, thereby causing material and production losses.
In another known technique, a liquid phase slurry aluminization process has been used for application of the aluminide coating. This involves directly applying the liquid phase slurry to the surface. Formation of the diffused aluminide is achieved by heating the part in a non-oxidizing atmosphere or vacuum at temperatures between 1600-2000° F. The heating melts the metal in the slurry and permits the reaction and diffusion of the aluminum into the substrate surface. However, the liquid phase slurry aluminization process suffers from the same drawbacks as the pack aluminization process. Generally speaking, both the directly applied pack and slurry pose difficulty due to the risk of fusing or sintering donor and activator to the part surface. Additionally, both techniques generate residual coating or bisque that is contained within the internal cavities and difficult to remove.
Other techniques include chemical vapor deposition (CVD) or vapor phase aluminide coating processes, whereby vaporized aluminide coating is generated external to the internal section of the part, and thereafter the vapor flow is directed into the internal sections of the part. CVD or vapor phase coatings are problematic as they require a constantly replenishing flow of aluminizing gas to the internal surface that is affected by component geometry and requires complex plumbing and gas control. Furthermore, conventional CVD and vapor phase coating processes have not proven capable of fully coating all of the required surfaces within the internal section of the part at the same rate as the external sections are being coated during a coating cycle. This can lead to uncoated surfaces. The problem of incomplete aluminide coverage along the internal sections has become even more problematic with components having increasingly complex geometries with advancement of various industry technologies, such as within the aerospace and energy sectors.
In view of the drawbacks with conventional aluminide coating processes, there is an unmet need for an aluminide coating process than can effectively coat internal surfaces with complex geometries in a simplified manner. Other advantages and applications of the present invention will become apparent to one of ordinary skill in the art.